The competing needs for accuracy and reduced computational overhead have presented problems in orbital parameter determination. Traditionally, either ephemeris data or information supplied by position measuring satellites have been independently used to determine orbital parameters such as spacecraft position and velocity.
Ephemeris data may be a tabulation of estimated orbital object positions at discrete instants in time which are used as a reference trajectory for predicting orbital parameters such as position and velocity. A large number of measurements and very sophisticated models can gene rate an extremely accurate ephemeris, but the computational overhead required for such accuracy makes it impractical to store it aboard orbital objects such as spacecraft. Additionally, even the most accurate ephemeris progressively loses accuracy with time because the forces influencing orbital motion are not precisely known. Specifically, unpredicted changes in the orbital perriod due to factors including atmospheric drag fluctuation tend to significantly affect the in-track component of reference trajectories.
Position information may also be used to determine orbital parameters. For example, a Global Positioning System ("GPS") receiver may be used to determine spacecraft position from signals transmitted from GPS satellites to the GPS receiver. Accuracy is adversely affected by factors such as the position and orientation of the spacecraft relative to the GPS constellation of satellites, GPS constellation position and timing errors, signal propagation errors, and internal receiver noise and timing errors. Additionally, GPS receivers are subject to Selective Availability ("S/A") which intentionally degrades signals and adds GPS satellite position and constellation timing errors. Each of these factors can cause positional error spikes in the position information.
Spacecraft position determination based solely upon GPS receiver outputs is also deficient because data output rates (typically 1 per second) are often too slow for spacecraft maneuvering. Additionally, GPS position estimates may not be available during maneuvers since GPS availability depends upon the antenna location and orientation of the spacecraft.
Sophisticated processes for GPS error correction such as those using full orbital state and Kalman filtering may be impractical because the extent of data collection, complex software and high precision arithmetic operations require high computational overhead and may prevent real-time processing. Additionally, such processes remain adversely affected by S/A errors since such errors are very difficult to predict.
Thus, there remains a need for real time orbital parameter determination with low computational overhead, high accuracy and S/A error mitigation.